Gravity Hurts (so Good)

Strange things can happen to the human body when
people venture into space -- and the familiar pull of gravity
vanishes.

August
2, 2001: Gravity hurts: you can feel it hoisting a loaded
backpack or pushing a bike up a hill. But lack of gravity hurts,
too: when astronauts return from long-term stints in space, they
sometimes need to be carried away in stretchers.

Gravity is not just a force, it's also a signal -- a signal
that tells the body how to act. For one thing, it tells muscles
and bones how strong they must be. In zero-G, muscles atrophy
quickly, because the body perceives it does not need them. The
muscles used to fight gravity --like those in the calves and
spine, which maintain posture-- can lose around 20 per cent of
their mass if you don't use them. Muscle mass can vanish at a
rate as high as 5% a week.

Above: Astronaut Bill
Shepherd prepares for a long stay on the International Space
Station with muscle-building exercises on Earth. [more]

For bones, the loss can be even more extreme. Bones in space
atrophy at a rate of about 1% a month, and models suggest that
the total loss could reach 40 to 60 per cent.

Blood feels gravity, too. On Earth, blood pools in the
feet. When people stand, the blood pressure in their feet can
be high -- about 200 mmHg (millimeters of mercury). In the brain,
though, it's only 60 to 80 mmHg. In space, where the familiar
pull of gravity is missing, the head-to-toe gradient vanishes.
Blood pressure equalizes and becomes about 100 mmHg throughout
the body. That's why astronauts can look odd: their faces, filled
with fluid, puff up, and their legs, which can lose about a liter
of fluid each, thin out.

But that shift in blood pressure also sends a signal. Our
bodies expect a blood pressure gradient. Higher blood pressure
in the head raises an alarm: The body has too much blood! Within
two to three days of weightlessness, astronauts can lose as much
as 22 percent of their blood volume as a result of that errant
message. This change affects the heart, too. "If you have
less blood," explains Dr. Victor Schneider, research medical
officer for NASA headquarters, "then your heart doesn't
need to pump as hard. It's going to atrophy."

The question is, do such losses matter?

Perhaps
not if you plan to stay in space forever. But eventually astronauts
return to Earth -- and the human body has to readjust to the
relentless pull of gravity. Most space adaptations appear to
be reversible, but the rebuilding process is not necessarily
an easy one.

Above: Astronaut Susan
Helms on Earth (left) and on board the International Space
Station (right).

"Each of the parameters have their own normal recovery
time," says Schneider. Blood volume, for example, is typically
restored within a few days. "Astronauts get thirsty when
they come back," Schneider explains, "because their
body says, you don't have enough blood in your blood vessels,
and that causes the messengers to say, drink more. [Also, the
body doesn't] urinate as much."

Muscle, too, can be recouped. Most comes back "within
a month or so, "although it might take longer to recover
completely. "We normally say that it takes a day [of recovery
on Earth] for each day that somebody's in space," says Schneider.

Bone recovery, though, has proven problematic. For a three
to six month space flight, says Schneider, it might require two
to three years to regain lost bone -- if it's going to come back,
and some studies have suggested that it doesn't. "You really
have to exercise a lot, says Schneider. "You really
have to work at it."

According
to Dr. Alan Hargens, recently of NASA Ames and now a professor
of orthopedics at the University of California San Diego medical
school, it is important to keep astronauts in good physical condition.
"You want the crew members to function normally when they
come back to Earth ... and not have to lie around for long periods
of rehabilitation," he says.

And Earth isn't the only planet that astronauts might visit.
One day humans will journey to Mars -- a six-month trip in zero-G
before they disembark on a planet with 38% of Earth's gravity.
"[We'll have to maintain] those astronauts at a fairly
high level of fitness," explains Hargens. "When they
get to Mars, there won't be anyone to help them if they get into
trouble." They will need to be able to handle everything
themselves.

Exercise is the key. But exercising in space differs from
exercising on Earth. Here, gravity's pull automatically provides
a resistive force that maintains muscles and bones. "[In
space] even if you do the same amount of work that you were doing
down here on Earth, you miss that gravity component," says
Schneider.

Various
devices have been developed to mimic the help that gravity provides.
One Russian experiment provides resistance by strapping jogging
cosmonauts to a treadmill with bungee cords. But that particular
combination has not yet proven effective in preventing bone loss
-- perhaps because it cannot provide sufficient loads. "The
straps are so uncomfortable that the cosmonauts can only exercise
at 60 to 70 per cent of their body weight, says Hargens.

There's also IRED, a NASA-developed Interim Resistive Exercise
Device. IRED consists of canisters that can provide more than
300 pounds of resistance for a variety of exercises. IRED's effectiveness
is still being monitored, says Schneider.

Yet another promising device attempts to mimic gravity even
more closely. Hargens and his colleagues are developing a Lower
Body Negative Pressure (LBNP) device, a chamber that contains
a treadmill, and that relies, says Hargens, on the suction of
an ordinary vacuum cleaner. "We've found," he says
"that we can provide body weight by applying negative pressure
over the lower body."

The device, explains Hargens, prevents much of the loss of
cardiovascular function and of muscle. It also seems to be effective
in reducing some indices of bone loss. One reason is that the
LBNP allows astronauts to exercise with an effective body weight
between 100% and 120% of what they would feel on Earth. Another
is that -- unlike any previous exercise device -- it restores
the blood pressure gradient, increasing blood pressure to the
legs.

There's growing evidence, Hargens says, that the body's systems
interact with each other. For example, "you can't just put
high loads on the bone and then expect it to recover if you're
not taking care of the blood flow to that bone as well."

Scientists aren't yet sure how gravity "signals"
the body to keep bones and muscles strong. "We know that,
somehow, gravity is converted from a mechanical signal to a chemical
signal -- and we know a lot about these chemical signals,"
says Schneider. The mechanical signals, though, remain a mystery.

Below: No pain, no gain! Astronaut Charles
Conrad Jr., commander of the first manned Skylab mission,
wipes perspiration from his face following an exercise session
on the bicycle ergometer during Skylab training at JSC. [more]

Solving
these problems, says Schneider, could lead to better therapies
for people who aren't using gravity properly here on Earth. Aging
is the perfect example. Zero-G living mimics closely the effects
of old age. Like astronauts, the elderly fight gravity less.
They're more sedentary, which triggers the loop of muscle atrophy,
bone atrophy, and lower blood volume.

If researchers can identify the signals that generate strong
muscles and bones, it might be possible "to get new pills
and do exercises" that would trigger those signals here
on Earth.

"We've just begun to do research ... looking at the changes
that can happen to humans," says Schneider. "There
are so many wonderful questions."

And the answers? They're waiting for us ... up there in space,
where the absence of weight reminds us that gravitation isn't
all bad. Sometimes it's a struggle, our daily contest with gravity,
but now we know the struggle is good!